Method for producing high density steel powders

ABSTRACT

A process for producing high density steel powders for Powder metallurgy is disclosed, wherein the molten stream of low carbon steel or low carbon alloy steel is atomized by high pressure water jet or inert gas jet to be powders, and after drying, the powders are heated in such inert gas as nitrogen or argon, whereby the reduction, decarburization and softening of the powders are simultaneously carried out.

United States Patent [1 1 [111 Kondo et al. June 3, 1975 \[54] METHOD FOR PRODUCING HIGH 3,746,584 7/1973 Takeda et al 75/.5 BA

DENSITY STEEL POWDERS [75] Ilnventors: Yoshikazu Kondo, Yokohama; Prim l y Exammer-L. Dewayne Rutledge ljvmsuo Ohhon Kudammsu both of Assistant ExaminerArthur J. Steiner apan Attorney, Agent, or F irm-Armstrong, Nikaido & \[73] Assignee: K020 Yoshizaki, Tokyo, Japan Wegner [22] Filed: ]Dec. 26, 1973 [21] Appl. No: 428,013

[57] ABSTRACT [30] \Foreign Application Priority Data Dec. 25 l972 Japan 47- 129379 A Process for Producing high density Steel Powders for Powder metallurgy is disclosed, wherein the molten 52 1 g 143 12 75 5 75 5 stream of low carbon steel or low carbon alloy steel is 75/2 1 atomized by high pressure water jet or inert gas jet to 51 tm. m B22f 9/00 be Powders, and after drying, the Powders are heated [58] Field of Search rs/.5 c, 211, 213. .5 BA; in Such inert gas as nitrogen or argon, whereby the 43 12 duction, decarburization and softening of the powders are simultaneously carried out. [56] References Cited llJNITED STATES PATENTS 14 Claims, 4 Drawing Figures 13,732,092 Ifill973 Wieland, llr. et al. 75/.5 BA

METHOD FOR PRODUCING HIGH DENSITY STEEL POWDERS DETAILED EXPLANATION OF INVENTION The present invention relates to a method for producing steel powders. comprising that the molten stream of low carbon steel or low carbon alloy steel is atomized by high pressure water jet or inert gas jet (hereinafter called water atomization or gas atomization) to be powders, and after drying, said powders are heated in such inert gas as nitrogen or argon, whereby the reduction, decarburization and softening of said powders are simultaneously carried out. It is the object of the present invention more economically and more safely to produce steel powders, having oxygen and carbon contained in a sufficiently small quantity to be soft, and besides, such a high density that there is almost not any pore or void in the powder, so that the density becomes high after compaction forming to be suitable for powder metallurgy, such as sintering or powder forging.

According to a conventional method, the low carbon steel powders, obtained by water atomization of molten steel, are heated in reducing gas, such as hydrogen gas or dissociated ammonia gas in order to soften and reduce the oxide formed on the surface of the powder due to the reaction with water at the time of atomization. Thus, reduction and softening may be achieved. In this case, there is used a method, in which the decarburization reaction of the carbon, contained in the steel powder can be simultaneously carried out by the moisture, which is generated by the reaction of hydrogen gas and oxides.

According to a conventional method for reduction by heating in the reducing gas, having hydrogen as the main ingredient, the tendency of the welding powder particles becomes greater due to heating at a higher temperature, and it becomes difficult to be broken up to the as atomized particle size after annealing, and the restoring by a comminuting operation takes much time. The comminution after reduction is impossible, so that it is not allowed to perform reduction at a higher temperature.

Accordingly, it is impossible to make the reactions of reduction, decarburization and softening more rapidly by heating at a higher temperature. Further, there is a problem in reducibility for the powder containing elements which are difficult to be reduced, because it is not possible to heat at a higher temperature.

Moisture is generated in the reaction of reduction by hydrogen gas, so that the gas is consumed and the dew point of the gas is raised to bring an oxidation atmosphere. Therefore, it is necessary to keep the dew point low by giving a large amount of gas. Naturally, the quantity of gas to be used becomes large. There is such a danger as gas leakage or gas explosion in case of gas, having hydrogen as the main ingredient. It caused anxiety always for the safety of human life and apparatus.

There is industrially used a continuous belt-type hoxizontal furnace or inclined furnace with muffle. But the quantity of gas to be used must be increased in order to prevent the gas explosion due to the permeation of air. And a large quantity of gas are given in the inlet and outlet of the furnace. Therefore, there is such a problem that the expense of gas becomes considerably large.

As for the obtained powder, it is the present condition that there is earnestly asked for the powder, having a higher density.

The method of the present invention is to solve the 5 defects of or to improve the above mentioned conventional method as well as to obtain the steel powder, having such a high density as equivalent to or to exceed that of the conventional method.

The inventive method is not such an ordinary method that the reduction of the oxide film, on the surface of steel powder due to the water atomization, as well as decarburization and softening are carried out by heating in a reducing atmosphere of hydrogen or mainly hydrogen, but the reduction as well as decarburization and softening are performed by a heat treatment in such inert gas as nitrogen or argon.

The inventive method which carried out the reduction of the oxide by inert gas seems apparently to be imposible. However, it was discovered as an experimental fact that the reducing reaction in the treatment of heating in inert gas can proceed at such a high velocity as to make an industrial performance possible, by the existence of a small quantity of carbon contained as impurity element or alloying element in low carbon steel powder, regardless of the low percentage of carbon.

It seems to be a phenomenon peculiar to powders that the reduction velocity is high regardless of such a low percentage of carbon. The additional experiments were carried out under various conditions to get the test results concerning the quantative relation between oxygen and carbon, contained in low carbon steel powders and taking part in the reaction in case of reduction by heating in inert gas.

Thus, the quantity of oxgen is kept small in the low carbon steel powder, subjected to water atomization, and the quantities of oxygen and carbon, contained in the powder, are given with the relation, obtained by the above-mentioned test results, whereby there has been developed a method for producing steel powders, having small quantities of oxygen and carbon and such a high density that at the time of compaction forming, the green density of the compact is equivalent to or higher than that of the conventional method, so that it is suitable for powder metallurgy, such as sintering or powder forging.

That is to say, the inventive method is characterized in that low carbon steel, having 0.02 to 0.38 percent, preferably 0.02 to 0.26 percent of carbon, or low carbon alloy steel, having alloying elements, such as Cu, Ni, Mo, Co, Cr, Mn, V, W etc. and having 0.02 to 0.38 percent, preferably 0.02 to 0.26 percent of carbon, is melted and the stream of the thus-obtained molten steel is atomized by high pressure water jet or high pressure inert gas jet to be tiny powders; said low carbon steel powder or low carbon alloy steel powder, in which the oxygen content after drying is made 0.20 to 0.50 percent, preferably 0.20 to 0.35 percent (it is represented by the percentage by weight of oxygen, decreased by reduction in the powder due to heating in H gas at 1,050C for 60 min. This value is hereinafter described as the quantity of oxygen or the oxygen content in this specification) and the carbon content and the oxygen content in the powder are given with the relation to meet the equation:

(K 0.375 to 0.75; shows the quantity of oxygen (7: by weight), decreased in the reaction in case of reduction by heating in inert gas; C% shows the necessary quantity of carbon (70 by weight) to be consumed in the reaction in case of reduction by heating in inert gas), being treated in heat at 600 to 1,250C in inert nitrogen gas or inert argon gas, whereby the powder can be reduced so as to make the oxygen content below 0.20 percent, preferably below 0.15 percent in the powder, and decarburized so as to make the carbon content below 0.02 percent, preferably below 0.01 percent, and simultaneously softened.

According to the present invention it is important that the oxygen content of the powder after water at omization or gas atomization should be made as small as possible between 0.20 and 0.50 percent, preferably between 0.20 and 0.35 percent, and kept constant.

The oxygen content of the steel powder, subjected to water atomization under the ordinary conditions, is about 0.8 to 3 percent. But the oxygen content can be kept in the steel powder within the limits of 0.20 to 0.50 percent, and further, within the narrow limits of 0.20 to 0.35 percent, by the increase of the flow of jet water at the time of water atomization, by making the water tank air-tight for atomization and by introducing nitrogen and argon so that the water atomization may be carried out in an inert atmosphere.

The larger the oxygen content after atomization becomes, the more difficult it is to obtain the high density powder, having no pore. After oxygen is taken away by reduction from the surface of the powder, the traces of dispersed oxygen remain in the form of pores and the surface of the powder becomes a porous layer.

The larger the contents of oxygen and carbon after atomization become beyond the upper limit, the greater the variety of the quantities of oxygen and carbon, contained in the powder after the treatment for reduction becomes. It becomes difficult for controlling to get the aimed value.

Considering these points, the upper limit of the oxygen content of the powder is determined to be 0.5 percent after atomization.

In case of gas atomization, the lower limit may be still lowered according to the conditions of production, while in case of the present method of water atomization, it is fairly difficult to make the oxygen content below 0.20 percent after atomization, so that the lower limit is determined to be 0.20%. Of course, the oxygen content become 0.20 percent gives good results.

FIG. 1 shows an optical microstructure of etched low cross section of carbon steel powder, in which the oxygen content is 0.295% and the carbon content is 0.103 percent after water atomization and which is treated by a reduction annealing at 900C for 1 hour in nitrogen gas according to the method, shown in Example 1 of the present invention. It proves that the powders have a high density without almost any pore.

FIG. 2 shows the cross section of powders, unetched of which the oxygen content after atomizing is 1.63 percent, subjected to a reduction annealing in a hydrogen at 900C for one hour. It proves the existence of porous powders with pores, made in the traces of oxygen, which is reduced and dispersed.

The thus-obtained atomized steel powder contains only a small amount of oxygen, but it is necessary to make the oxygen content still smaller than 0.20 per cent, preferably smaller than 0.15 percent in order to give the suitable properties for powder metallurgy.

In order to reduce the oxygen content enough to the objective value below the above mentioned value by the heat treatment in inert gas, and to keep the carbon content in the powder to lower level below 0.02 percent after heat treatment, carbon content in the atomized powder must keep within the ranges of 0.02 to 0.38 percent, preferably 0.02 to 0.26 percent and the quantities of carbon and oxygen in the atomized powder must meet the relation, shown by the before mentioned equation 1 In the equation (1 0% shows the necessary quantity of carbon by weight) to be consumed in a reducing reaction. Assuming that the quantity of carbon, contained in the powder after atomization, is A% and the carbon content is B% after this powder is treated in heat in inert gas, it can be expressed by C% (A B) 0% shows the quantity of oxygen by weight), reduced in a reducing reaction by the heat treatment. Assuming that the oxygen content of the powder is D% after atomization, and the oxygen content is E% after this powder is treated in heat in inert gas, it can be expressed by 0% (D E) K is a constant and takes a value between 0.375 and 0.750, and it changes within the above-mentioned limits according to the temperature of heating in inert gas and the flow rate of inert gas.

If the equation (1) is rewritten according to the above it may be expressed by the equation (2) (AB)%=K-(D-E)% (K 0.375 to 0.75)

These relation shall be clearly explained for help of understanding. If the objective quantity of oxygen (5%) of the powder is 0.15 percent and the objective quantity of carbon (3%) is 0.010 percent after the reduction treatment is carried out in inert gas, and assuming that K 0.51. If the oxygen content (D%) of the powder is 0.30 percent after atomization (before reduction), and the carbon content (A%) of the same is 0.086 percent, the equation (I) or (2) is satisfied and it is possible to obtain the powder, having the objective quantities of oxygen and carbon.

When the relation between the quantities of oxygen and carbon in the powder after atomization is not the relation to satisfy the equation l) or (2) and the quantity of carbon is short, the carbon content of the powder can be sufficiently lowered after the heat treatment, but the oxygen content cannot be decreased to the objective value, so that the object of the present invention cannot be achieved.

On the contrary, if the carbon content of the powder after atomization is excessive over the relation, shown in the equation (l or (2) in relation to the oxygen content, the oxygen content of the powder after the heat treatment can be sufficiently decreased, but the carbon content cannot be decreased to the objective low value.

As above, the upper and lower limits of the quantity of carbon, contained in the powder after atomization are determined corresponding to the upper and lower limits of the oxygen content of the powder.

As for the relation between the quantity of oxygen 0% to be reduced by heating in nitrogen gas and the necessary quantity of carbon C% to be consumed in the powder for reduction, an example is shown in FIG. 3.

ln this case, the value of K is within the extent of 0.40 to 0.63, but the value of K changes according to the temperature of heating and the flow rate of inert gas.

FIG. 4 shows the relation between the carbon content of the powder after atomization (before the heat treatment) and the carbon content 7: after the heat treatment, in the test examples. in which various kinds of powders, having 0.27 to 0.3l percent of oxygen contained after atomization, are treated in heat at 900C for 1 hour in the atmosphere of nitrogen gas, whereby the quantity of oxygen to be contained in the powder is made below 0.15 percent after the heat treatment.

It becomes known from the drawing that the quantity of carbon to be contained in the powder after atomization may be about 0.13 percent or below in order that the carbon content is made below 0.02 percent after the heat treatment, and further, that the carbon content becomes below 0.005 percent after the heat treatment if the quantity of carbon to be contained in the powder after atomization is made 0.06 to 0.08 percent.

The reaction of reduction in inert gas according to the present invention is the reaction between the oxide, formed on the surface of the powder mainly by atomization, and the carbon, contained in the powder particles in a few amount. It seems that the carbon, contained as a solid solution or a carbide compound in the steel powder after atomization, is rapidly diffused to the surface of the powder and reacts on the oxygen, contained in the oxide layer to be a gas mixture of carbon monoxide and carbon dioxide, so that the reducing reaction can proceed. And this reaction proceeds quickly. For example, the reducing reaction in nitrogen gas for low carbon steel powders, having 0.075 percent of carbon and 0.30 percent of oxygen contained, proceeds to the almost finished condition by soaking at 900C for l0 min, and the quantity of carbon contained in the powder after reduction becomes 0.001 percent and the oxygen content becomes 0.15 percent. It is known that the reduction proceeds in a remarkably short time as compared with the time, necessary for the softening of the powder and the velocity is enough for the performance in the industry. Further, the comparison concerning the decarburization rate is shown in Example 4. The carbon content is decreased to the objective value far more quickly than that of heating in gas, of which the main ingredient is hydrogen. The powder, having the carbon content decreased sufficiently, can be surely obtained in a short time.

Thus, the reduction and decarburization of the powder can be carried out in such a short time as similar to or less than that of the case of hydrogen being used.

The heating in inert gas is carried out within the limits of 600 to l,250C.

The effect of the heating in inert gas is that the property to be welded by heating at a high temperature is weak as compared with the case of heating in hydrogen gas or in a reducing gas, having hydrogen gas mainly contained, as explained hereinafter in the paragraph of the effect of the present invention.

Comparing for examplev the case that the low carbon steel powder, subjected to water atomization, is heated at 1,000C for 60 min, in nitrogen gas and the case that the same is heated in dissociated ammonia gas, containing 75 percent of hydrogen, the ratio of the quantity of particles, smaller than 325 meshes after the lump solid powder subjected to the heat treatment is broken up in a stamp mill, to the quantity of particles, smaller than 325 meshes containing in the powder after water atomization is almost percent in case of heating in nitrogen gas and the original condition can be fully restored, while in case of heating in dissociated ammonia gas the ratio is lowered to 50 percent and only the half can be restored to the original state. Thus the comminution ability in the case of heating in dissociated ammonia gas becomes considerably worse.

Therefore, according to the inventive method, the heating at a high temperature can be more easily performed. Then the reduction rate and the softening rate can be increased. The upper limit of heating is made 1,250C, because it is favourable for the reducing reaction to raise the heating temperature when such ele ments as Cr or Mn, difficult to be reduced, are contained.

On the contrary, if the temperature is too low, the reduction rate is low and the reduction becomes difficult. It becomes also difficult to achieve the purpose of softening. However, it is sometimes not necessary so much to perform the softening treatment of the powder. The lower limit is made 600C in consideration of the above-mentioned.

The heating time is the time, during which the reduction and softening proceed. It is about 10 min. or more, but it is not particularly limited.

Next, as for the quantity of nitrogen gas, as shown in Example 2, explained later, the object can be fully achieved by the flow of i0 l/h or less per 1 kg of the powder to be treated. Even in case a horizontal type of continuous furnace, it is not necessary to flow an excess of gas in order to prevent explosion as necessitated in the case of hydrogen gas or dissociated ammonia gas. Thus, the quantity of gas to be used can be drastically decreased.

It is feared that nitrogen may be increased in the low carbon steel powder, obtained by heating at a high temperature in inert nitrogen gas. However, it was confirmed that the quantity of nitrogen was not increased in the powder after the heat treatment as shown in the following examples, and was not different from that of the case of using hydrogen gas.

The quantity of oxygen to be contained in the low carbon steel powder after atomization can be controlled to a. low value and an almost constant value by keeping the conditions of atmization constant. And the quantity of carbon, contained in the powder, can be controlled within the narrow limits by controlling the carbon, contained in molten steel, so that the inventive method can be easily performed.

Such aluminium oxide, magnesium oxide, calcium oxide or silica, as difficult to be reduced, has the tendency to be mixed from the refractory materials of a furnace or others when the raw material is molten. Therefore, it goes without saying that it must be kept as little as possible.

As above mentioned, the oxygen content is expressed in this specification as a percentage by weight of oxygen decreased in the powder by reduction with hydrogen gas in case of heating at l,050C for 60 min. in hydrogen gas, namely hydrogen loss. 0

However, as the quantity of carbon contained in the steel powder increases, the weight is decreased due to the decarburization with moisture, generated in the reaction of hydrogen gas and oxygen at l,050C, and also due to the decarburization with the reaction of hot hy drogen gas and carbon contained in the powder. This decrease is contained in the hydrogen loss. Therefore, this must be amended to be the oxygen content, by analysis of the quantities of carbon, contained in the powder before and after the hydrogen loss is measured.

Further, in application of the present invention, atomized steel powders or atomized alloy steel powders, having 0.20 to 0.50 percent, preferably 0.20 to 0.35 percent of oxygen, less than 0.38 percent, preferably less than 0.26 percent of carbon and besides the carbon content, showing a value smaller than the quantity of carbon, necessary for reduction, as shown by the equation (1), are added and mixed with carbon or graphite particles in the quantity of carbon, insufficient to meet the above-mentioned equation (1). And then, by heating at 600 to 1,250C in inert gas, the oxygen, contained in the powder is reduced to less than 0.2 percent, preferably less than 0.15 percent and the carbon is decarburized to less than 0.02 percent, preferably less than 0.01 percent. The softening may be simultaneously carried out.

In this case, however, the carbon powders are sometimes mixed not uniformly. The reaction velocity for reduction with carbon powders is lower than that of the case of carbon, contained in the steel powders and there may be sometimes produced an unevenness in reduction due to the ununiformity of mixing. Attention must be paid, accordingly, to mix sufficiently small carbon powders fully.

When machine parts are manufactured by sintering or powder forging, there is generally adopted a method, comprising that low carbon steel powders or low carbon alloy steel powders are added and mixed with a necessary quantity of carbon powders and then, formed by compaction. However, when the green density of a compact is not required to be so much high at the compaction forming, for example in case of the pre-forming for powder forging, it is often favourable that carbon is added as alloying element in the melting process of raw materials, so that it may be contained in the raw material powder.

ln such a case, it is also possible to produce the aimed steel powder in application of the inventive method.

That is to say, atomized steel powders or atomized alloy steel powders are made to contain 0.20 to 0.50 percent, preferably 0.20 to 0.35 percent of oxygen and also the maximum 0.6 percent of carbon in excess of the quantity of carbon (C%), necessary to reduction as shown in the equation l The reduction and softening of powders are carried out by heating these powders in inert gas, whereby the carbon steel powders, having a sufficiently small quantity of oxygen and the carbon content of the maximum 0.6 percent, can be easily manufactured.

In this case, the quantity of carbon to be added as an alloying element is within the extent to be ordinarily used for powder metallurgy. The upper limit of the quantity of carbon after the reduction treatment is 0.6 percent.

The effect of the present invention is that as beforementioned, the property of the powder to be welded in a heat treatment is weaker as compared with that of the case of heating in gas consisting of mainly hydrogen, so that the breakability after heating is better. Accordingly, the breaking operation may be a light worlt. Because of a weal: property to be welded by heating, it is possible to treat in heat at a higher temperature than that of the case of having hydrogen gas as the main ingredient. Even in case there is contained such metal ox ides of Cr and Mn as difficult to be reduced, it is possible to treat at a higher temperature to increase the reductibility. And also, it is favourably possible to heat at a higher temperature in order to accelerate the reac tions of reduction, decarburization and softening.

Further, there is no fear of gas explosion and the effect is great for the safety of a human life and apparatus, because of inert gas being used, differently from a reducing gas, having hydrogen as the main ingredient, such as hydrogen or dissociated ammonia gas.

The inert gas does not take part in the reducing reaction and is not consumed at all at the time of heating. Therefore, the quantity of inert gas to be used may be remarkably smaller than that of the reduction with a reducing gas, having hydrogen as the main ingredient. in case of a horizontal continuous furnace being used, a large amount of excessive gas must be used in order to prevent explosion when the gas contains hydrogen chiefly. But according to the present invention there is not such necessity and the quantity of gas to be used can be considerably decreased.

The reducing reaction proceeds quickly, and also, the decarburization of carbon, contained in the powder, can be surely carried out more quickly as compared with the case of gas, having hydrogen as the main ingredient.

The powder, obtained according to the present invention, has few pores and a high density. The density of a compact shows such high value at the time of compaction forming as equivalent to or higher than that of the powder, obtained by the conventional method as shown in the examples. The compact has a good edge stayability, namely a high Ratra value and is excellent for powder metallurgy.

Further, the inventive method has a great effect in that the reducing power is greater than that of the reaction with hydrogen gas, because of a reducing reaction with carbon, contained a little in the powder particle. The method has also predominance in this point.

BRIEF EXPLANATION OF DRAWINGS FIG. 1 shows a photograph (x250) of optical microstructure of the cross sention of the powder, obtained in Example I. The crystal structure was developed by etching with 5% of alcohol nitrate.

FIG. 2 is an optical microphotograph (x250) of the cross section of the comparative powder, without etching.

FIG. 3 shows some test results concerning the relation between the quantity of oxygen in the powder, reduced by heating in inert gas, and the necessary quantity of carbon, contained in the powder to be consumed in reduction.

FIG. t shows some test results concerning the relation between the quantity of carbon, contained in the powder after atomization and the quantity of carbon,

contained in the powder after a heat treatment in nitrogen gas.

The examples shall be presented according to the present invention in the following.

EXAMPLE 1 With use of an electric furnace, low carbon steel scraps were melted. The quantity of carbon was so controlled as to be 0.10 percent.

The molten steel was continuously discharged down from a tandish, having a hole of 12 mm d), by gravity into an atomizing tank, in which water was filled in the lower part. From a waterjet nozzle, installed below the tandish, water was impinged to the molten steel stream under the pressure of 65 kg/cm at the flow of 950 l/min. The molten steel was pulverized to be powders.

The atomizing tank is closely sealed. The air in the tank was replaced by nitrogen gas before water atomization. The atmosphere of nitrogen gas was maintained during the atomization. The quantity of oxygen, contained in the powder after atomization, was controlled to be 0.30 percent.

The chemical composition of the atomized low carbon steel powder was carbon 0.103%, silicon 0.01%, monganese 0.09%, phosphorus 0.015%, sulphur 1,1013%, oxygen 0.295% and nitrogen 0.001%. After coarse particles were removed, the size distribution was as follows:

+80 imesh 1.5%

"+10 +100 do. 6.1% -l +150 do. 13.3% 150 +200 do. 20.4% 1200 +250 do. 24.0% 1250 +325 do. 12.8% 325 do. 219% 100 kg of this powder were heated at 850C for 1.5 hours in a belt conveyor type of inclined horizontal continuous maffle furnace with the nitrogen gas, having the purity above 99.9 percent and the dew point of 30C, being flowed in the quantity of 10m /h.

The powder particles looked to be slightly welded each other after heating, but were fragile and easily restored to the original size by a light smashing.

After heating, the powder contained 0.002 percent of carbon, 0.036 percent of oxygen and 0.001 percent of nitrogen. The apparent density was 3.10g/CC and the flow rate, 134.9sec/50g. The handness of the powder was 87 of micro-Vickers. The oxygen content and the carbon content were sufficiently small without any increase of nitrogen. The hardness was sufficiently decreased and the powder was softened.

The optical microstructure of the powder after treatment in heat teaches that the density is high without any pore and the recrystallization proceeds fully due to the heating as shown in FIG. 1.

About 6g of the thus-obtained powders were taken up and filled in a metal mold. With zinc stearate being used for die lubrication, the compaction forming was carried out at t/cm The powders were formed in a cylindrical shape of 10mm in diameter and about 10mm in height.

The green density of the compact was a high density of 7.08g/cc. The edge stayability (Ratra value) showed a sufficient value of 0.46 percent.

Thus, the powder, obtained by the inventive method, had the sufficiently small contents of oxygen and carbon, and the green density of the compact was so high that the powder was possessed of the characteristics and the high density, excellent for powder metallurgy.

By the way, in case that the powder, produced by a conventional method, was heated at 800 to 900C in the composition of dissociated ammonia gas, containing percent of hydrogen, the density of the compact, formed under the pressure of 5t/cm was 6.75 to 7.00g/cc.

EXAMPLE 2 With use of an electric furnace. low carbon steel scraps were melted. The carbon content of the molten steel was adjusted to 0.07 percent with the aim of 0.27 percent for the oxygen content after atomization. The molten steel was atomized with the flow of water of 1,050 liters min. in the atmosphere of nitrogen gas by the similar method to that of Example 1.

The chemical composition of the obtained powder was as follows:

carbon 0.074%, silicon 0.01%. manganese 0.10%, phosphorus 0.013%. sulphur 0.014%, nitrogen 0.001%, oxygen 0.273%,

This powder was sieved after drying. Those of mesh were taken away.

The size distribution was mesh 5.3 percent, 100 +325 mesh 71.6 percent, 325 mesh 23.1 percent.

1 kg. of this powder was put in a muffle-type heating furnace and treated in heat 900C for one hour with the nitrogen gas, having the dew point of 30C, being flowed at the flow rate of 100 liters per hour.

After treatment in heat, the powder contained 0.13 percent of oxygen, 0.002 percent of carbon and 0.001 percent of nitrogen. The contents of oxygen and carbon can be made sufficiently small.

In case of heating at 900C for 1 hour, the flow of nitrogen gas was made to change to 10 liters, 25 liters or 50 liters per hour. In this case, the carbon content was within the extent of 0.001 to 0.003 percent and the oxygen content, within the extent of 0.14 to 0.12 percent. The object could be fully achieved even if the flow of nitrogen gas was decreased.

EXAMPLE 3 Low carbon steel scraps were melted in an electric furnace and so adjusted that the quantities of copper, molybdeum, nickel and carbon may become the aimed value. The powder was produced by water atomization under the almost similar conditions as shown in Example 2.

After coarse particles were taken away, the size distribution was as follows:

+80 mesh 5.2%

-80 +100 do. 102% 100 do. 26.5% 150 +200 do. 25.5% 200 +250 do. 7.6% 250 +325 do. 8.4% 325 do. 16.6%

The values according to the chemical analysis showed carbon 0.091%, manganese 0.04%, copper 3.90%, molybdenum 0.45%, nickel 0.74%, silicon trace, nitrogen 0.001% and the oxygen content was 0.261%, so that the aimed value was satisfied.

100 kg of this powder were treated in heat at 850C for one hour in an continuous belt-type heating fur- 11 l nace. having a muffle employed, by flowing the nitrogen gas, having the purity above 99.9 percent and the dew point of 30C, at the flow rate of llm /h.

The powder after treatment in heat was broken up extremely easily. According to the value of the chemical analysis carbon was 0.002%, oxygen, 0.053% and nitrogen 0.001%. The alloy steel powder, having the sufficiently small contents of oxygen and carbon, was obtained.

The apparent density of this powder was 3.39g/cc; the flow rate, 22.7 sec/50g; the green density of the compact, 6.61g/cc in case of compaction under the pressure of t/crn with zinc stearate being used for die lubrication; and the edge stayability (Ratra value), favourably 0.38 percent.

This powder was mixed with carbon powders and in addition of 1 percent of zinc stearate, was formed by compaction at 4.5t/cm to be a compact, having 58mm in diameter and about 40 mm in height. The compact was sintered at 1,120C for 30 min. after dewaxing in dissociated ammonia gas.

This specimen was heated at 900C for 30 min. in nitrogen gas and then, forged under the pressure of about l4t/cm with use of a mechanical press.

After forging, the density was 7.92 and the carbon content, 0.16 percent.

This specimen was heated at 900C for 30 min. in nitrogen gas, and then, after oil quenching, tempered at 550C for 1 hour to make the Rockwell C hardness to be 30. It showed the tesile strength of 105.4 kg/mm the elongation of 14.8%, the reduction of 50.1% and the impact strength of 9.4 kg-mlcm The powder forged articles, having excellent mechanical properties, were obtained A sealed Jominy test piece, having the diameter of 6 mm, was cut out of the powder forged article. The quench hardenability was estimated on the basis of the quench hardenability testing method. JlS G0561.

The Rockwell C hardness was 46.5 at 1.5 mm distant from the quenched end, 44.5 at 5 mm, 39.5 at 10 mm, 31.5 at mm and 30.5 at 40 mm, and showed and excellent quench hardenability, exceeding the upper limit value, namely 46 at 1.5 mm distant from the quenched end, 42 at 5 mm, 31 at 11 mm, 26 at 20 mm and 23 at 40 mm, of the quenched Jominy band of chromiummolybdeum steel, SCM 21, the conventional structural steel, JlS G4105, which was the aim for this example.

EXAMPLE 4 The molten steel, having the quantities of copper, molybdeum, nickel and carbon adjusted to the prescribed quantity, was melted with use of an electric furnace. The powder was produced by water atomization in a similar method to that of Example 1.

The chemical analytical values of the obtained powder showed 0.123% of carbon, 0.06% of manganese, 2.65% of copper, 0.51% of molybdenum, 0.61% of nickel and the oxygen content was 0.321%. After +80 mesh was removed, the size distribution of the powder was as follows:

+100 mesh 7.5% -100 +150 do. 21.3% -150 +200 do. 25.2% 200 +250 do. 9.8% 250 +325 do. 11.6% -325 do. 24.6%

50 kg of this powder were continuously put in a belt conveyer type of inclined continuous maffle furnace with the nitrogen gas, having the purity above 99.9 percent and the dew point of 30C and flowed at the flow rate of l3m"/h. The powder was treated in heat at 850C for 1.5 hours.

The powder, treated in heat, had the carbon content of 0.002 and the oxygen content of 0.12 percent. The quantities of carbon and oxygen were decreased to a sufficiently low level.

For the comparison, with use of the same furnace. 50 kg of the same powder were continuously put in the furnace, in which the dissociated ammonia gas, having the dew point of -30C was flowed at 13m /h. The powder was treated in heat at 850C for 1.5 hours. The powder, treated in the dissociated ammonia gas, showed the carbon content of 0.065% and the oxygen content of 0.12percent. It is known that the carbon content is not yet sufficiently decceased and decarburization property is inferior to that of the inventive method and there is a great difference between both methods.

EXAMPLE 5 Low carbon steel scraps were melted in an electric furnace and the aimed value of the carbon content was made 0.045 percent so that the carbon content may be decreased to less than the value, shown in C% K 0% of the relation between the quantities of carbon and oxygen. The powder was produced by water atomization similarly to Example 2.

The chemical composition of this powder was 0.045% of carbon, 0.01% of silicon, 0.10% of manganese, 0.015% of phosphorus, 0.014% of sulphur, 0.001% of nitrogen and the oxygen content was 0.270%.

1 kg of this powder was added with 0.04% of carbon powder and mixed for one hour in a mixer. And then, the powder was heated at 950C for one hour in a muffle furnace with the high purity nitrogen gas, having the dew point of 30C, being flowed at the flow rate of 50 l/h.

The carbon content was 0.008%, the oxygen content, 0.13% and nitrogen, 0.001% in the powder after the heat treatment. The contents of carbon and oxygen were decreased to a low value.

What is claimed is:

1. A method for producing high density steel powders, suitable for powder metallurgy; characterized in that atomized steel powders and atomized alloy steel powders, having 0.02 to 0.38 percent by weight of carbon and 0.20 to 0.50 percent by weight of oxygen contained, and wherein the quantitative relation of carbon to oxygen can meet the equation C% K'O% (K=0.375 to 0.75), are heated at 600 to 1,250C in inert gas, whereby reduction as well as decarburization and softening are simultaneously carried out.

2. A method for producing high density steel powders, suitable for powder metallurgy; characterized in that atomized steel powders and atomized alloy steel powders, having 0.20 to 0.50 percent by weight of oxygen and less than 0.38 percent of carbon contained besides, having a lower percentage of carbon than the relation shown by the equation C% [(0% (K=0.375 to 0.75), are heated at 600 to 1,250C in inert gas after fine powders of carbon are added and mixed by the quantity of carbon, insufficient for said equation,

whereby reduction as well as decarburization and softening are simultaneously carried out.

3. The method of claim 1 wherein the atomized steel powders and atomized alloy steel powders contain 002 to 0.26 percent by weight of carbon and 0.20 to 0.35 percent by weight of oxygen.

4. The method of claim 2 wherein the atomized steel powders and atomized alloy steel powders contain 0.20 to 0.35 percent by weight of oxygen and less than 0.26 percent by weight of carbon.

5. The method of claim 1 wherein said alloy steel powders contain at least one alloying element of Cu. Ni, Mo, Co, Cr, Mn, W and V.

6. The method of claim 1 wherein the inert gas is nitrogen or argon gas.

7. The method of claim 1 wherein the dew point of said inert gas employed is lower than C.

8. The method of claim 1 wherein the heating time in inert gas is more than 10 minutes.

9. The method of claim 1 wherein the powders are atomized in the presence of a nitrogen inert gas atmosphere.

10. The method of claim 2 wherein said alloy steel powders contain at least one alloying element of Cu, Ni, Mo, Co, Cr, Mn, W and V.

11. The method of claim 2 wherein the inert gas is nitrogen or argon gas.

12. The method of claim 2 wherein the dew point of said inert gas employed is lower than -30C.

13. The method of claim 2 wherein the heating time in inert gas is more than 10 minutes.

14. The method of claim 2 wherein the powders are atomized in the presence of a nitrogen inert gasatmosphere. 

1. A METHOD FOR PRODUCING HIGH DENSITY STEEL POWDERS, SUITABLE FOR POWDER METALLURGY; CHARACTERIZED IN THE ATOMIZED STEEL POWDERS AND ATOMIZED ALLOY STEEL POWDERS, HAVING 0.02 TO 0.38 PERCENT BY WEIGHT OF CARBON AND 0.20 TO 0.50 PERCENT BY WEIGHT OF OXYGEN CONTAINED AND WHEREIN THE QUANTITATIVE RELATION OF CARBON TO OXYGEN CAN MEET THE EQUATION C% = K.O% (K=0.375), ARE HEATED AT 600* TO 1,250*C IN INERT GAS, WHEREBY REDUCTION AS WELL AS DECARBURIZATION AND SOFTENING ARE SIMULTANEOUSLY CARRIED OUT.
 1. A method for producing high density steel powders, suitable for powder metallurgy; characterized in that atomized steel powders and atomized alloy steel powders, having 0.02 to 0.38 percent by weight of carbon and 0.20 to 0.50 percent by weight of oxygen contained, and wherein the quantitative relation of carbon to oxygen can meet the equation C% K.O% (K 0.375 to 0.75), are heated at 600* to 1,250*C in inert gas, whereby reduction as well as decarburization and softening are simultaneously carried out.
 2. A method for producing high density steel powders, suitable for powder metallurgy; characterized in that atomized steel powders and atomized alloy steel powders, having 0.20 to 0.50 percent by weight of oxygen and less than 0.38 percent of carbon contained besides, having a lower percentage of carbon than the relation shown by the equation C% K.O% (K 0.375 to 0.75), are heated at 600* to 1,250*C in inert gas after fine powders of carbon are added and mixed by the quantity of carbon, insufficient for said equation, whereby reduction as well as decarburization and softening are simultaneously carried out.
 3. The method of claim 1 wherein the atomized steel powders and atomized alloy steel powders contain 0.02 to 0.26 percent by weight of carbon and 0.20 to 0.35 percent by weight of oxygen.
 4. The method of claim 2 wherein the atomized steel powders and atomized alloy steel powders contain 0.20 to 0.35 percent by weight of oxygen and less than 0.26 percent by weight of carbon.
 5. The method of claim 1 wherein said alloy steel powders contain at least one alloying element of Cu, Ni, Mo, Co, Cr, Mn, W and V.
 6. The method of claim 1 wherein the inert gas is nitrogen or argon gas.
 7. The method of claim 1 wherein the dew point of said inert gas employed is lower than -30*C.
 8. The method of claim 1 wherein the heating time in inert gas is more than 10 minutes.
 9. The method of claim 1 wherein the powders are atomized in the presence of a nitrogen inert gas atmosphere.
 10. The method of claim 2 wherein said alloy steel powders contain at least one alloying element of Cu, Ni, Mo, Co, Cr, Mn, W and V.
 11. The method of claim 2 wherein the inert gas is nitrogen or argon gas.
 12. The method of claim 2 wherein the dew point of said inert gas employed is lower than -30*C.
 13. The method of claim 2 wherein the heating time in inert gas is more than 10 minutes. 